1. Field of the Invention
The present invention relates in its most general sense to refrigeration, and more particularly to defrosting controls for the evaporators of refrigerated food display cases. More particularly, the control means constituting the present invention is of the automatic type and is associated with those defrosting means or devices categorized by the industry as "demand defrost" devices, that defrost evaporators only when defrost is actually needed, as distinguished from defrost devices that utilize time clocks and defrost their associated evaporators at predetermined, timed intervals without regard to actual need for defrosting on the part of said evaporators.
2. Description of the Prior Art
Prior to the present invention, it has been common practice in food supermarkets or similar establishments, to utilize, extensively, refrigerated food display cases each of which is provided with its own evaporator (or sets of evaporators), together with means for defrosting said evaporator or sets thereof.
In a typical supermarket installation, there will be a variety of such cases, according to the particular food products that are to be refrigerated and displayed therein. As a result, some of food-retaining areas of the refrigerated display cases are maintained at temperatures distinctly different from the temperatures required for the products-containing areas of other cases. The result is that some of the refrigerated display cases require defrosting more often, than do others installed in the same commercial establishment.
Commonly, the defrost cycles of evaporators of the type described above have been initiated and terminated by means of time clocks such as shown, for example, in U.S. Pat. No. 3,464,226. However, the accumulation of ice and frost on the coils and fins of an evaporator tends to vary markedly from time to time, and from day to day, by reason of sensitivity of the refrigeration system to changes humidity, and the activity of customers in opening or closing doors or reaching into the cases.
The frequency of defrosting operations controlled by a time clock thus may be either more or less than that required for efficient operation of the equipment. This has been long noted by those working in the art, and as a result, various types of defrost control devices have been proposed, that respond not to passage of a predetermined period of time, but rather, to an actual need of the associated evaporator or set of evaporators to be defrosted. These are usually referred to as "demand defrost" controls, typical examples of which are found in U.S. Pat. Nos. 3,282,065; 3,355,904; 3,374,643; 3,464,224; and 3,453,837.
Although demand defrost control devices have been used to advantage in the art, they intrinsically possess characteristics that tend to lessen their desirability as compared to defrost systems utilizing time clocks. The advantages of a demand defrost control system over one utilizing time clocks may be reduced or completely lost, if they result in overloading of the refrigeraton system. If, for example, the system employs electrical defrosting devices individual to the several evaporators or sets of evaporators, excessive demands may be made upon the electrical power circuits involved should a number of the evaporators call for defrost at any one time.
If the defrost system be of the type that utilizes hot gas from a compressor or compressors common to the several evaporators or sets of evaporators, a call for defrost by a number of the evaporators at any one time produces the undesirable result of rendering wholly inadequate the amount of gaseous refrigerant that is returned to the compressor, as well as the supply of liquid refrigerant available to those evaporators that are still operating on a refrigeration cycle.
The state of the art as it existed prior to completion of the present invention, accordingly, may be generally assessed as including, for a series of evaporators or sets thereof, automatic defrost controls utilizing time clocks, which have the advantage of permitting settings that preclude overloading of electrical or hot gas defrost systems at any given time, but which have the disadvantage of initiating defrosting of evaporators whether they need it or not; and on the other hand, defrost controls of the "demand" type, which have the advantage of initiating defrost of evaporators only when defrost is acutally needed, but which have the disadvantage of overloading electrical power circuits or critically affecting the refrigeration cycles of evaporators that do not at the moment require defrost.